1. Field of the Invention
The present invention relates generally to a new ceramic composition for immobilizing actinides, in particular plutonium, thorium, and uranium. The present invention relates particularly to a ceramic composition comprising pyrochlore, brannerite and rutile. The ceramic composition is attractive for immobilization of excess plutonium because of its extremely low leachability, the existence of natural mineral analogues that have demonstrated actinide immobilization over hundreds of millions of years in wet geological environments, and the high solid solubility of actinides in the ceramic thus providing a relatively low overall waste volume. Incorporation of plutonium into ceramic provides a form that is relatively easy to store but renders retrieval of the plutonium to be more difficult than other immobilized forms.
2. Description of Related Art
Because of their extreme durability, ceramic forms have been studied extensively since the late 1970s for the immobilization of high level waste (HLW). The material called Synthetic Rock (SYNROC) is a titanate-based ceramic composed approximately of 30% zirconolite, 30% hollandite, 30% perovskite and 10% rutile. In the HLW application, actinides partition into the zirconolite and perovskite phases. U.S. Pat. No. 4,274,976 (Ringwood), U.S. Pat. No. 4,329,248 (Ringwood), U.S. Pat. No. 4,645,624 (Ramm, Ringwood) and U.S. Pat. No. 4,808,337 (Ramm et al.) disclose the immobilization of HLW in synthetic rock.
A form of synthetic rock to immobilize spent fuel (SYNROC-F) was reported by S. E. Kesson and A. E. Ringwood, "Safe Disposal of Spent Nuclear Fuel ", Radioactive Waste Management and the Nuclear Fuel Cycle, Vol. 4(2), pp. 159-174, October 1983. This form of SYNROC consisted of 90 wt % uranium pyrochlore, 5 wt % hollandite and 5 wt % rutile.
A variant of SYNROC-F, namely SYNROC-FA, was reported by A. G. Solomah, P. G. Richardson and A. K. McIlwain, "Phase Identification, Microstructural Characterization, Phase Microanalyses and Leaching Performance Evaluation of SYNROC-FA Crystalline Ceramic Waste Form ", Journal of Nuclear Materials 148, pp. 157-165, 1987. This form of SYNROC consisted of uranium pyrochlore, perovskite, uraninite and hollandite.
A cold press and sinter process is used in the production of mixed oxide (MOX) fuel from uranium and plutonium. The MOX process uses pressing pressures in excess of 20,000 psi and sintering temperatures of 1,700 deg C in a 4% H.sub.2 atmosphere on a 24 hour cycle. Because the final product is to be used as fuel, impurities in the feedstock cannot be tolerated.